An electronic image pickup apparatus, for example, a television camera, typically comprises solid-state image pickup devices, such as charge coupled device (CCD) image sensors and an imaging optical system which functions to form images of an object on light receiving surfaces of these image pickup devices. The image pickup devices of this type typically comprise a large number of picture elements which are disposed in lattice patterns and configured so as to spatially sample, at discontinuously distributed picture elements, the images of the object formed on the light receiving surfaces of the image pickup devices. These image pickup devices function to convert the sampled light intensities into corresponding video electrical signals that may be transmitted to a display unit, such as a monitor television set, that reproduces the images represented by the video signal.
When the images of the object contain components which have spatial frequencies exceeding a Nyquist frequency limit for the image pickup devices, the image pickup apparatus equipped with the image pickup devices for spatially sampling the images of the object allows spurious signals, such as moiré or aliasing to be produced, thereby remarkably degrading quality of the images which are reproduced on the display unit. For this reason, a typical imaging optical system used in the image pickup apparatus of this type is configured so as to exhibit an effect to limit spatial frequency response so that components having frequencies in the vicinity of the Nyquist frequency limit are eliminated from spatial frequency components contained in the images of the object (optical low pass effect). The optical low pass effect is obtained, for example, by forming dualized images with a quartz filter disposed in the imaging optical system or blurring the images of the object with a phase filter.
Japanese Utility Model Kokai Publication No. SHO63-24,523 discloses a method to form dualized images by disposing a wedge-shaped prism in the imaging optical system so that a portion of a light bundle passes through this prism to obtain the optical low pass effect. Japanese Patent Kokai Publication No. SHO47-38,001 discloses a method to form ring-shaped blurred images with a conical prism disposed in the imaging optical system. Japanese Patent Kokai Publication No. SHO63-6,520 discloses a method to blur the images by producing spherical aberration of higher orders in the imaging optical system.